1. Field of the Invention
The present invention relates to a wireless IC device, and particularly relates to a wireless IC device preferably for use in an RFID (Radio Frequency Identification) system.
2. Description of the Related Art
In recent years, an RFID system has been put into practical use as an article information management system, which includes: a reader/writer that generates an induction field; and an RFID tag (also referred to as a wireless IC device) that is attached to an article, and non-contact communication using the induction field is established between the reader/writer and the RFID tag to transmit predetermined information therebetween. The RFID tag is composed of a wireless IC chip that has stored predetermined information therein and processes a predetermined wireless signal; and an antenna that performs transmission/reception of high-frequency signals. The RFID tag is attached to various articles as management objects (or their packing materials) when used.
As RFID systems, a HF-band RFID system using the 13 MHz band and a UHF-band RFID system using the 900 MHz band are typically known. Particularly, with the UHF-band RFID system, the communication distance is relatively long (the communication area is relatively wide), and it is possible to collectively perform reading/writing of a plurality of tags. Thus, the UHF-band RFID system is regarded as a promising system for article management.
As wireless IC devices for use in the UHF-band RFID system, for example, Japanese Patent Nos. 4069958 and 4561932 describe wireless IC devices in which a power supply circuit substrate is interposed between a wireless IC element (wireless IC chip) and an antenna element. In these wireless IC devices, the frequencies of transmission/reception signals are practically determined by a power supply circuit provided on the power supply circuit substrate. Thus, it is possible to easily realize stable high-frequency characteristics without being greatly influenced by the size of the antenna element (radiating plate) and the surface area of its periphery.
However, in each wireless IC device described above, when it is attempted to expand its frequency band, the power supply circuit becomes complicated, some coil elements and capacitor elements have to be incorporated thereinto, and the necessity to ensure isolation between these elements occurs. Thus, there is a problem that the size of the power supply circuit substrate is increased.